1. Field of the Invention
The present invention relates to an optical disk apparatus for recording information on an optical disk by projecting a light beam onto the optical disk.
2. Description of the Related Art
For high density recording of information on an optical disk, the pit-edge recording method is known in which information "1" is recorded at both the leading edge and trailing edge of one recording pit, as contrasted with the conventional pit-position recording method in which information "1" is recorded in one pit position.
FIG. 1 is a diagram explaining the information recording and regeneration signals and the states recording pits. The data word of information to be recorded is converted, for example, by "2/7" coding, into recording data WD (RZ modulation) as shown in FIG. 1(a) in the case of the pit-position recording method, and into recording data WD (NRZI modulation) as shown in FIG. 1(b) in the case of the pit-edge recording method, to record the information on an optical disk. In the recording of information "1", the information "1" is recorded during one pulse period in the case of FIG. 1(a). On the other hand, in the case of FIG. 1(b), the information "1" is recorded at both the leading edge and trailing edge of a recording pulse. In other words, in the case of FIG. 1(a), a single "1" can be recorded with one pulse, while in the case of FIG. 1(b), two "1"s can be recorded with one pulse.
In the pit edge recording, a laser diode is driven by a laser driving signal LP having the same pulse width as that of the pulse of the recording data WD shown in FIG. 1(b), and the light beam emitted from thus driven laser diode is projected onto the optical disk. The area of the optical disk illuminated by the light beam is heated. The area in the recording layer which exceeds the Curie temperature by heating is to be magnetized in the direction of an externally applied magnetic field, thereby forming a recording pit WP whose length corresponds to that of the laser drive signal LP, as shown in FIG. 1(d). However, immediately after the start of illuminating the light beam, the temperature of the optical disk surface near the leading edge of the pit is lower, so that the area that exceeds the Curie temperature is restricted; therefore, the pit width W of the recording pit is narrower immediately after the start of illuminating the light beam. Thereafter, as the heat of preceding light beam is accumulated and thus the temperature of the optical disk surface rises, the pit width W increases, thus forming the recording pit in a teardrop shape.
On the other hand, when regenerating the recorded information, a light beam is projected onto the optical disk, and the light reflected from the optical disk is detected to obtain a regeneration signal RD as shown in FIG. 1(e). The regeneration signal RD is then compared with a present threshold level Vth to obtain a pit detection signal DP as shown in FIG. 1(f). The pit detection signal DP is used to regenerate the recorded information. However, if the recorded pit WP is of a teardrop shape, the level of the regeneration signal RD at the leading edge of the recorded pit, i.e. the narrower portion is low, and the regeneration level gradually increases as the pit width increases as shown in FIG. 1(e). As a result, the period that exceeds the threshold level Vth becomes narrower than the pulse width of the recording data DW. In other words, the pulse width of the pit detection pulse DP shown in FIG. 1(f) is significantly shorter as compared with the pulse width of the recording data WD shown in FIG. 1(a) or (b) corresponding to information " 1". This prevents accurate recording and regeneration of the information.
An information recording method aiming to overcome the above problem is published in Proc. Int. Symp. on Optical Memory, 1987 Japanese Journal of Applied Physics, Vol. 26 (1987) Supplement 26-4. FIG. 2 is a timing chart of recording and regeneration signals and recording pits according to that method. The recording data WD shown in FIG. 2(a) and (b) have the same patterns as the recording data WD shown in FIG. 1(a) and (b), respectively. As shown in FIG. 2(c), a correction prior to recording is made to the laser driving signal LP in such a manner that the pulse level thereof is significantly increased for a prescribed period from the leading edge of the pulse thereby increasing the power of the laser. As a result, the temperature of the optical disk surface quickly rises immediately after the start of illuminating the light beam, allowing the recording pit WP to be formed in an oval shape, as shown in FIG. 2(d), having a wide pit width W from the leading edge. Therefore, when regenerating the recorded information, the regeneration signal RD shown in FIG. 2(e) is obtained from the light reflected from the optical disk, which sharply rises. Consequently, the period during which the regeneration signal RD exceeds the threshold level Vth becomes longer than in the case of the teardrop-shaped recording pit shown in FIG. 1(d), so that the pit detection pulse DP has a wider pulse width as shown in FIG. 2(f). The pulse width of the pit detection pulse DP is approximately equal to the pulse width of the recording data WD shown in FIG. 2(a) or (b), thus ensuring accurate regeneration of the information recorded on the optical disk.
However, in order to raise the laser diode power by increasing the level of the laser driving signal LP for a prescribed period on the leading edge side, as described above, it requires the use of a laser diode having a high power, the resulting problem being that the reliability of the optical disk apparatus is impaired and life of the apparatus is reduced.